1. Field of the Invention
The present invention relates to safety devices that automatically cut-off the burner operation of a hot water boiler. More specifically, the present invention relates to the type of boiler used in residential and light commercial heating applications that include a control system for monitoring both the temperature and level of the water in the boiler. The system allows these attributes to be measured through a single probe inserted into the boiler through a single tapping.
2. Description of Related Art
In conventional boilers of the type used in residential and light commercial heating the water level is monitored with a low water cutoff (LWCO) sensor. When the water level in the boiler drops below the level of the low water cutoff sensor, the burner is turned off until the water level is brought back up to a safe level. These controls are well known in the art.
One example is U.S. Pat. No. 6,390,027, issued to Lyons and Murray, entitled, “CYCLE CONTROL SYSTEM FOR BOILER AND ASSOCIATED BURNER,” which is incorporated by reference. In the '027 design, a cycle control system is used with a boiler to determine the presence of an adequate level of fluid within the boiler.
In operation, an LWCO has a probe that extends into the boiler through a single tapping. Generally, the probe has two electrically conductive surfaces that are isolated from each other. An electrical signal is provided to one of these conductive surfaces. When the water level is above the level of the probe, the circuit between the conductive surfaces is closed by virtue of the conductivity of the water surrounding the probe. When the water level falls below the probe, there is no conductivity between the metal conductive surfaces. Thus, the circuit is open, and the control detects a low water condition.
Another component of monitoring boiler systems is information concerning the temperature of the water in the boiler. There are many temperature control systems in the art currently used to monitor water temperature in a boiler. Commercially available temperature controllers include, for example, the Honeywell L7224U Aquastat Relay. In these devices, a temperature sensing thermistor is inserted into a hollow well. The hollow well is then inserted within a tapping in the boiler and the boiler is filled with water. The thermistor is connected to a central control unit. The central control unit monitors the temperature gradient, and is typically programmed to shut down the burner to prevent the water in the boiler from exceeding a preset limit. The central control unit may also be programmed to turn the burner ON to maintain a minimum boiler temperature.
In U.S. Pat. No. 5,340,019, issued to Bohan, Jr., et al., entitled, “ELECTRONIC AQUASTAT IN IMMERSIBLE CAPSULE,” a liquid immersible electronic aquastat is taught in which a temperature responsive element and substantially all associated electronic circuitry are arranged on a circuit board within a tubular capsule of liquid impervious material. The capsule or well houses a thermocouple, while conducting heat energy from the surface of the well to the temperature sensor.
A need exists to combine the two safety functions of monitoring for low water cutoff and temperature measurement in a single probe with supporting control circuitry to allow it to perform in existing boiler tappings, thus eliminating the need to drain the boiler to insert a new tapping.
In U.S. Pat. No. 5,111,691, issued to John, et al., entitled, “CONDUCTANCE/THERMAL LIMIT CONTROL APPARATUS AND METHOD,” a temperature probe is taught which mounts in a liquid container. The probe has a conductance electrode coupled to a conductance control circuit. A temperature sensor is combined with this low water cutoff probe. This control, however, does not allow for the ability to replace the sensor of an existing aquastat such as the Honeywell devices described above without draining the boiler and possibly the entire heating system. Furthermore, this design only provides high temperature limit with no provision to turn the burner on to maintain a minimum boiler temperature, or to control the circulator pump on a call for heat.
One problem in the industry has been the reluctance to accommodate multiple tappings for water cutoff probes and temperature sensor probes. This requires expensive redesigns of boiler castings to accommodate a second hole in the boiler wall for the additional probe. It is desirable to combine the two measurement functions in a single probe, which can be inserted into a single well. It is further desirable to construct a probe/well design that can accomplish the multiple measurements in a single device that is interchangeable with existing well designs currently available in the industry. In this manner, it is not necessary to provide a new tapping or to drain existing boilers currently in operation in order to incorporate the present invention. In addition, it is desirable for the present invention to maintain a minimum boiler temperature.
Another problem that occurs is when the control circuitry is set to maintain a minimum water temperature and the temperature sensor is not in the boiler. In the prior art, the control circuitry would incorrectly determine that the water temperature is too low, and try to run the boiler, causing an unsafe, high temperature condition. This is generally referred to as a “run-away boiler” condition. In the present invention, if the dual probe sensor is connected to the control circuitry but not inserted into the well, the control circuitry will sense a low water condition, and not allow the burner to fire.